This application is based on and incorporates herein by reference Japanese Patent Application No. 2001-227680 filed on Jul. 27, 2001.
The present invention relates to a power supply circuit utilized for a vehicular control device and continues power generation after switch turn-off.
In a power supply circuit used for a vehicular control device, large capacitance is required for an output capacitor in a constant voltage circuit. Moreover, connectors in the power supply circuit may cause problems due to inadequate connection.
A power supply circuit that counters the above problem is proposed in U.S. Pat. No. 6,084,384 (JP-A-11-266547). As shown in FIG. 4, this power supply circuit includes a battery 61, the first power supply line BATT, the second power supply line VB and a switching device 62, which includes an ignition switch. The first power supply line BATT is continuously supplied with power from the battery 61. The second power supply line VB is supplied with power from the battery 61 only when the switching device 62 is closed. A primary constant voltage circuit 63 is connected to the second power supply line VB and an auxiliary constant voltage circuit 64 is connected to the first power supply line BATT. The constant voltage circuits 63 and 64 supply power to the third power supply line DL.
The primary circuit 63 includes a transistor 63a and a constant voltage control IC 63b. The auxiliary circuit 64 includes a transistor 64a and a constant voltage control IC 64b. A halt control circuit 65 is provided to continue operations of the auxiliary circuit 64 for a predetermined period after power supply to the second power supply line VB is cut off.
In the power supply circuit, power is normally supplied from the primary circuit 63 to the third power supply line DL. After the switching device 62 is opened, the power is supplied from the auxiliary circuit 64 to the third power supply line DL. If the second power supply line VB is momentarily shut down, a constant voltage is supplied by the auxiliary circuit 64. The output voltage of the auxiliary circuit 64 is adjusted lower than that of the primary circuit 63.
However, when the power supply circuit is implemented on an IC chip, a voltage drop in output voltage may occur. When the switching device 62 is opened and constant voltage generation by the primary circuit 63 is halted, the auxiliary circuit 64 starts a constant voltage generation. During the period between the time that the switching device 62 is opened and the time that the auxiliary circuit 64 starts providing a sufficient voltage, a voltage drop may occur. The dropped voltage may trigger a low voltage reset.
As shown in FIG. 5, when the switching device 62 is switched from ON (closed) to OFF (opened), the primary circuit 63 enters the non-operating state and the auxiliary circuit 64 enters the operating state. It takes for a while until the output voltage of the auxiliary circuit 64 rises to a sufficient level. As a result, the output voltage of the power supply circuit drops during that period.
In some vehicular control devices, a plurality of constant voltage circuits are provided in a power supply circuit to generate constant voltages in each section. In recent years, the constant voltage is lowered to cut back power consumption of an onboard battery. For instance, a power supply circuit that produces different constant voltages for sensors and a CPU has been introduced. In such a power supply circuit, constant voltage generation for each constant voltage circuit is controlled by opening and closing a switching device such as an ignition switch. Since requirements for reduction in power consumption and for constant voltage variation will increase, it is preferable that the circuit configuration is more simplified.
The present invention therefore has an objective to provide a power supply circuit that implements desired constant voltage generation with a simple configuration.
The present invention has another objective to simplify the configuration of the power supply circuit that outputs a variety of constant voltages.
The power supply circuit of the present invention includes a constant voltage circuit provided in a power supply line that is continuously supplied with power from a power source. When a power supply switching device is closed, the constant power supply circuit performs constant voltage generation. The power supply circuit also includes a timer circuit. The timer circuit controls the constant voltage circuit to continue the constant voltage generation for a predetermined period (allowable period) after the switching device is opened.
If an instantaneous power interruption occurs and the interrupted period is shorter than the allowable period, the constant voltage generation continues. Therefore, the constant voltage is outputted without interruption. When the interrupted period reaches the allowable period, the constant voltage generation is halted.
The power supply circuit of the present invention does not require a large-capacitance output capacitor to handle the instantaneous power interruption. Since the power supply circuit of the present invention does not require the switching, the voltage drop is prevented. Therefore, a desired constant voltage generation by the simple power supply circuit is possible.